System and method for assigning a sub-channel in a BWA communication system

ABSTRACT

A method for requesting a Base Station (BS) to assign a sub-channel by a Subscriber Station (SS) in a Broadband Wireless Access (BWA) communication system including the SS and the BS, the BS assigning the sub-channel to the SS by a predetermined scheme. The method includes determining by the SS an assignment scheme of a sub-channel currently assigned to the SS and confirming channel quality indicator of the sub-channel assigned by the determined sub-channel assignment scheme, transmitting, to the BS, a codeword representing a sub-channel assignment scheme to be converted, when the SS determines a need to convert the current sub-channel assignment scheme, and receiving a sub-channel through the converted sub-channel assignment scheme according to the transmitted codeword.

PRIORITY

This application claims priority to an application entitled “System and Method for assigning Sub-channel in BWA Communication System” filed in the Korean Intellectual Property Office on Nov. 4, 2004 and assigned Serial No. 2004-89369, the contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and more particularly to a system and a method for assigning a sub-channel in a BWA communication system.

2. Description of the Related Art

Communication systems are now capable of providing voice services, data services, and various multimedia services. However, because the conventional voice-based communication system has a relatively small data transmission bandwidth and imposes high service charges on users, it is difficult to satisfy the increasing service demands of users.

In addition, with the development of communication industry and increase of user demands for an Internet service, it is more necessary to provide a communication system capable of satisfying the user demands.

Accordingly, a BWA communication system has been proposed, which has enough broadband to satisfy the user demands while efficiently providing an Internet service.

The BWA communication system integrates voice, low speed dates services and high speed data services with multimedia application services for high quality dynamic images, and then deliver these integrated services. The BWA communication system is capable of connecting to a Public Switched Telephone Network (PSTN), a Packet Data Serving Node (PDSN) network, an Internet network, an International Mobile Telecommunication (IMT)-2000 network, an Asynchronous Transfer Mode (ATM) network, etc., through wireless media using a broadband including 2 GHz, 5 GHz, 26 GHz, 60 GHz, etc. Further, the BWA communication system is capable of supporting a channel transmission rate of more than 2 Mbps.

Commonly, the BWA communication system is classified into a Broadband Wireless Local Loop (BWLL), a Broadband Radio Access Network (BRAN), and a high speed Wireless Local Area Network (WLAN) according to a terminal's mobility (stationary or mobile), communication environments (indoor or outdoor), and a channel transmission rate.

The radio access scheme of the BWA communication system has been standardized by an Institute of Electrical and Electronics Engineers (IEEE) 802.16 standardization group. The IEEE 802.16 standardization group is currently establishing an IEEE 802.16d standard and an IEEE 802.16e standard as a standard for providing a wireless broadband Internet service to stationary terminals or mobile terminals. In particular, research is being performed to support a high speed service capable of ensuring mobility and various levels of Quality of Service (QoS) in a BWA communication system such as a WLAN communication system and a wireless Metropolitan Area Network (MAN) communication system. Representative communication systems of the BWA communication system are the IEEE 802.16d communication system and the IEEE 802.16e communication system (hereinafter commonly referred to as IEEE 802.16d/e communication systems).

The IEEE 802.16d/e communication systems utilize an Orthogonal Frequency Division Multiplexing (OFDM) scheme/an Orthogonal Frequency Division Multiplexing Access (OFDMA) scheme in order to support a broadband transmission network for a physical channel of the wireless MAN system. However, the IEEE 802.16d communication system considers only a single cell structure and stationary Subscriber Station (SS), which means the system does not accommodate the mobility of the SSs at all. The IEEE 802.16e communication system accommodates the mobility of an SS in the IEEE 802.16d communication system. Herein, an SS having the mobility will be referred to as a Mobile Station (MS).

Because the IEEE 802.16d/e communication systems have a wide data transmission bandwidth as compared to radio technology for an existing voice service, they can transmit mass storage data in a short amount of time and efficiently use channels by sharing all user channels. Further, because the IEEE 802.16d/e communication systems ensure QoS, users can receive various services of different qualities according to the characteristics of a service.

In the IEEE 802.16d/e communication systems, because all users connected to a Base Station (BS) share and use a common channel and intervals, for which each user uses the channel, are assigned by the BS in each uplink and downlink frame, the BS must inform each user of uplink and downlink access information so that each user can separately use the channel.

Accordingly, the IEEE 802.16d/e communication systems classify channel information into uplink and downlink channel information, define information for each channel by Type, Length and Value (TLV), insert the defined information for each channel into a Downlink Channel Descriptor (DCD) message and a Uplink Channel Descriptor (UCD) message, and periodically transmit the DCD and UCD messages to all users, thereby informing SSs of characteristic information for channels.

FIG. 1 is a diagram schematically illustrating a conventional frame structure of an IEEE 802.16d or IEEE 802.16e communication system.

Referring to FIG. 1, a frame is classified into a downlink (DL) sub-frame 100 and an uplink (UL) sub-frame 140. The downlink sub-frame 100 includes a preamble field 110, an MAP field 120, and a downlink (DL) burst field 130. The preamble field 110 is a field through which synchronization signals, i.e., a downlink preamble sequence, are transmitted in order to acquire synchronization between a BS and SSs. The MAP field 120 is a field through which a downlink-MAP message is transmitted, and the DL burst field 130 is a field through which downlink data targeting the SSs are transmitted. The downlink sub-frame 100 includes sub-channels assigned through a normal sub-channel assignment scheme and a band Adaptive Modulation and Coding (AMC) sub-channel assignment scheme.

The uplink sub-frame 140 includes an uplink (UL) burst field through which uplink data targeting the BS are transmitted from the SSs.

The IEEE 802.16 d/e communication systems assign resources to each user through an uplink and a downlink by the sub-channel, which is a set of specific sub-carriers. The band AMC sub-channel assignment scheme adaptively changes a modulation technique and a coding technique according to wireless environments in order to improve the data transmission efficiency. Further, because it's a basic algorithm for the band AMC sub-channel assignment scheme is already known to those skilled to the art, a detailed description will be omitted.

The normal sub-channel assignment scheme may include a Partial Usage Sub-Channel (PUSC) assignment scheme, a Full Usage Sub-Channel (FUSC) assignment scheme, an optional FUSC assignment scheme, an AMC permutation assignment scheme, etc. For an uplink, a PUSC, an optional FUSC, an AMC permutation, etc., exist. All other sub-channel assignment schemes excluding the band AMC sub-channel assignment scheme basically assign sub-carriers randomly scattered in an entire frequency domain to one sub-channel, thereby causing each user receiving sub-channels to acquire frequency diversity gain.

In the IEEE 802.16 d/e communication systems, a BS assigns Channel Quality Indicator Channels (CQICHs) for reporting its own Channel Quality Indicator (CQI) to the SSs, and each of the SSs informs the BS of its own channel conditions through the assigned CQICH. The CQICH is a uplink channel used when the SS measures and compares reception qualities of downlink pilot channels received from BSs included in an active set, selects one BS from which the SS is to receive downlink packet data, and feedbacks the reception qualities of the pilot channels to the BS. That is, the CQICH represents a channel assigned to the SSs for the CQI.

More specifically, in the IEEE 802.16 d/e communication systems, the BS assigns the CQICH to all SSs, and each SS may transfer the CQI of 5 bits or 6 bits to the BS through the CQICH. For example, because the CQI of 6 bits which may be transmitted from each SS through the CQICH corresponds to 2⁶, it is possible to use the total 64 codewords. From among the 64 codewords, 32 codewords are used in order to report a Carrier-to-Interference Noise Ratio (CINR) value of a channel to the BS. With the exception of the already defined codewords used for other purposes of the other 32 codewords not being used for reporting the CINR value, seven codewords have not yet been currently defined.

If an SS has received a sub-channel through the normal sub-channel assignment scheme, the SS converts an average CINR value for a whole bandwidth according to 32 (2⁵) levels for a 5 bit encoding, and transmits encoding results to a BS through the CQICH.

If an SS has received a sub-channel through the band AMC sub-channel assignment scheme, the SS selects five bandwidths having the best channel conditions, and transmits a differential value between CINR values for the selected bandwidths to the BS. When the differential value received from the SS is 1, the BS determines that a CINR value of the current frame is larger than a CINR value of the previous frame. However, when the differential value is 0, the BS determines that the CINR value of the current frame is smaller than the CINR value of the previous frame.

When the BS assigns sub-channels through the band AMC sub-channel scheme, it is necessary to report CINR values for each bandwidth of the SSs to which the sub-channels are assigned.

FIGS. 2 and 3 are flow diagrams schematically illustrating conventional signal flows for sub-channel assignment between a BS and an SS in wither an IEEE 802.16d or IEEE 802.16e communication system. More specifically, FIG. 2 is a flow diagram illustrating a BS transmitting a CQI request to the SS and assigning a sub-channel to the SS, and FIG. 3 is a flow diagram illustrating an SS requesting the BS to assign a sub-channel by transmitting a CQI to the BS.

Referring to FIG. 2, the BS 220 transmits a CQI Report Request (REP-REQ) message to the SS 210 in step 201. The SS 210 transmits a Report Response (REP-RSP) message including its own CQI to the BS 220 in response to the REP-REQ message in step 202. After receiving the REP-RSP message including the CQI from the SS 210, the BS 220 assigns a sub-channel through the band AMC sub-channel assignment scheme in step 203. After receiving the sub-channel from the BS 220 through the band AMC sub-channel assignment scheme, the SS 210 selects five bandwidths having the best channel conditions, and transmits a CQI including a differential value between CINR values for the five selected bandwidths, i.e., a Band AMC Differential CQI, to the BS 220 from a frame, after transmitting the REP-RSP message in step 204.

Referring to FIG. 3, when the SS 310 is to receive a sub-channel from the BS 320 through the band AMC sub-channel assignment scheme, the SS 310 transmits the REP-RSP message including its own CQI to the BS 320 without reception of the REP-REQ message. That is, the SS 310 informs the BS 320 that the SS 310 is to first receive the sub-channel through the band AMC sub-channel assignment scheme in step 301. The BS 320 assigns the sub-channel through the band AMC sub-channel assignment scheme in step 302.

After receiving the sub-channel from the BS 320 through the band AMC sub-channel assignment scheme, the SS 310 selects five bandwidths having the best channel conditions, and transmits a CQI including a differential value between CINR values for the five selected bandwidths, i.e., a Band AMC Differential CQI, to the BS 320 from a frame after transmitting the REP-RSP message in step 303.

When a BS assigns a sub-channel through the normal sub-channel assignment scheme, the BS assigns the sub-channel by using the downlink MAP message of the MAP field 120 in FIG. 1 without using separate signals through the normal sub-channel assignment scheme, and an SS having received the sub-channel from the BS through the normal sub-channel assignment scheme reports an average CINR value for a whole bandwidth to the BS. That is, when the SS receives the sub-channel through the band AMC sub-channel assignment scheme, the SS selects five bandwidths having the best channel conditions, and reports only a differential value between CINR values for the five selected bandwidths. However, when the SS receives the sub-channel through the normal sub-channel assignment scheme, the SS reports the average CINR value for the whole bandwidth to the BS.

Herein, in the IEEE 802.16 d/e communication systems, it may be impossible for a BS to normally receive the REP-RSP message transmitted from an SS for reception of the sub-channel through the band AMC sub-channel assignment scheme. When the BS has not normally received the REP-RSP message from the SS, the BS may determine the differential value between the CINR values for the five bandwidths having the best channel conditions, which is transmitted from a frame after the SS has transmitted the REP-RSP message, as the average CINR value for the whole bandwidth. Therefore, a problem may occur in the sub-channel assignment through the band AMC scheme and it may be difficult to provide a communication service to the SS. Further, the stability and reliability of the system may deteriorate.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been designed to solve the above and other problems occurring in the prior art. It is an object of the present invention to provide a system and a method for assigning a sub-channel according to a CQI in a BWA communication system.

It is another object of the present invention to provide a system and a method for stably assigning a sub-channel by using either a normal sub-channel assignment scheme or a band AMC sub-channel assignment scheme in a BWA communication system.

It is further another object of the present invention to provide a system and a method for a sub-channel assignment, which prevents a problem from occurring during conversion to either a normal sub-channel assignment scheme or a band AMC sub-channel assignment scheme in a BWA communication system.

In order to accomplish the above and other objects, according to an aspect of the present, there is provided a method for requesting a Base Station (BS) to assign a sub-channel by a Subscriber Station (SS) in a Broadband Wireless Access (BWA) communication system including the SS and the BS, the BS assigning the sub-channel to the SS by a predetermined scheme. The method includes the steps of: determining, by the SS, an assignment scheme of a sub-channel currently assigned to the SS; confirming channel quality indicator of the sub-channel assigned by the determined sub-channel assignment scheme; transmitting, to the BS, a codeword representing a sub-channel assignment scheme to be converted, when the SS determines a need to convert the current sub-channel assignment scheme; and receiving a sub-channel through the converted sub-channel assignment scheme according to the transmitted codeword.

According to another aspect of the present, there is provided a method for requesting by a Subscriber Station (SS) a sub-channel assignment through a second sub-channel assignment scheme different from a first sub-channel assignment scheme in a Broadband Wireless Access (BWA) communication system including the SS and a Base Station (BS), the BS assigning a sub-channel to the SS through the first sub-channel assignment scheme. The method includes the steps of: transmitting, by the SS, a first codeword to the BS through a dedicated control channel; transmitting a second codeword and channel quality indicator of the SS to the BS through the dedicated control channel when a channel quality indicator request message is received from the BS in response to the first codeword; and receiving a sub-channel through the second sub-channel assignment scheme according to the transmitted channel quality indicator and first codeword.

According to further another aspect of the present, there is provided a method for requesting by a Subscriber Station (SS) a sub-channel assignment through a first sub-channel assignment scheme different from a second sub-channel assignment scheme in a Broadband Wireless Access (BWA) communication system including the SS and a Base Station (BS), the BS assigning a sub-channel to the SS through the second sub-channel assignment scheme. The method includes the steps of: transmitting, by the SS, a codeword to the BS through a dedicated control channel; transmitting channel quality indicator of the SS according to the transmitted codeword; and receiving a sub-channel through the first sub-channel assignment scheme converted according to the transmitted channel quality indicator and the codeword.

According to still another aspect of the present, there is provided a method for assigning by a Base Station (BS) a sub-channel to a Subscriber Station (SS) through a predetermined scheme in a Broadband Wireless Access (BWA) communication system including the SS and the BS. The method includes the steps of: receiving, by the BS, a codeword representing conversion from a current sub-channel assignment scheme to another sub-channel assignment scheme; when channel quality indicator of the SS corresponding to the codeword is received, converting the current sub-channel assignment scheme to the another sub-channel assignment scheme according to the channel quality indicator and the codeword; and assigning a sub-channel through the converted sub-channel assignment scheme.

According to yet another aspect of the present, there is provided a system for assigning a predetermined scheme in a Broadband Wireless Access (BWA) communication system. The system includes: a SS for determines an assignment scheme of a sub-channel currently assigned to the SS, confirms channel quality indicator of the sub-channel assigned by the determined sub-channel assignment scheme, transmits a codeword representing a sub-channel assignment scheme to be converted, when the SS determines a need to convert the current sub-channel assignment scheme, and receives a sub-channel by the converted sub-channel assignment scheme converted according to the transmitted codeword; and a BS for receives the codeword from the SS, converts the current sub-channel assignment scheme according to channel quality indicator and the codeword, and assigns a sub-channel through the converted sub-channel assignment scheme.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram schematically illustrating a conventional frame structure of a BWA communication system;

FIG. 2 is a flow diagram illustrating a conventional signal flow for sub-channel assignment between a BS and an SS in a BWA communication system;

FIG. 3 is a flow diagram illustrating a conventional signal flow for sub-channel assignment between a BS and an SS in a BWA communication system;

FIG. 4 is a flow diagram illustrating a signal flow for sub-channel assignment between a BS and an SS in a BWA communication system according to an embodiment of the present invention; and

FIG. 5 is a flow diagram illustrating a signal flow for sub-channel assignment between a BS and an SS in the BWA communication system according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of the present invention will be described in detail herein below with reference to the accompanying drawings. In the following description, a detailed description of known functions and configurations incorporated herein will be omitted when it may obscure the subject matter of the present invention.

In general, the present invention proposes a system and a method for assigning a sub-channel in a Broadband Wireless Access (BWA) communication system, e.g., an Institute of Electrical and Electronics Engineers (IEEE) 802.16d communication system and an IEEE 802.16e communication system. Further, the present invention proposes a system and a method in which a Base Station (BS) assigns a sub-channel to a Subscriber Station (SS) in a BWA communication system.

In the BWA communication system according to an embodiment of the present invention, the BS assigns a Channel Quality Indicator Channels (CQICH) to the SS in order to report its own Channel Quality Indicator (CQI), and the SS informs the BS of its own CQI through the assigned CQICH. Then, the BS assigns a sub-channel according to the CQI of the SS transmitted through the CQICH. Herein below, the term “SS” includes both stationary SSs and SSs having mobility, i.e., MSs.

The present invention describes a system and a method in which a BS assigns the CQICH of 6 bits, an SS transmits its own CQI to the BS through the assigned CQICH, and the BS assigns a sub-channel by using a normal sub-channel assignment scheme, which is a first sub-channel assignment scheme, or a band Adaptive Modulation and Coding (AMC) sub-channel assignment scheme, which is a second sub-channel assignment scheme, according to the received CQI. The normal sub-channel assignment scheme represents an assignment scheme excluding the band AMC sub-channel assignment scheme, and may include a Partial Usage Sub-Channel (PUSC) assignment scheme, a Full Usage Sub-Channel (FUSC) assignment scheme, an optional FUSC assignment scheme, an AMC permutation assignment scheme, etc.

In the BWA communication system according to the present invention, the BS assigns the CQICH of 6 bits to the SS, and the SS informs the BS of its own CQI through the CQICH of 6 bits. The CQI that may be transmitted through the CQICH of 6 bits may use a total of 64 codewords. From among the 64 codewords, 32 codewords are used to report a Carrier-to-Interference Noise Ratio (CINR) value of a channel. With the exception of the already defined codewords used for other purposes of the other 32 codewords not being used for reporting the CINR value, seven remaining codewords have not yet been defined.

Accordingly, the present invention newly defines and uses three codewords of the seven remaining undefined codewords. More specifically, the present invention newly defines and uses the three codewords, and assigns a sub-channel by using either the normal sub-channel assignment scheme, which is a first sub-channel assignment scheme, or the band AMC sub-channel assignment scheme, which is a second sub-channel assignment scheme. For convenience of description, the three codewords will be referred to as a first codeword C1, a second codeword C2, and a third codeword C3, respectively.

The first codeword C1 is transmitted from an SS to a BS through a CQICH, which is a dedicated control channel. That is, the first codeword C1 is transmitted from an SS to request the BS to convert a sub-channel assignment scheme from the normal sub-channel assignment scheme, i.e., the first assignment scheme, to the band AMC sub-channel assignment scheme, i.e., the second assignment scheme, and to assign a sub-channel. That is, the BS having received the C1 from the SS through the CQICH converts the sub-channel assignment scheme from the normal sub-channel assignment scheme to the band AMC sub-channel assignment scheme, and then assigns the sub-channel to the SS.

The second codeword C2 is transmitted from the SS to the BS through the CQICH, which is the dedicated control channel when the BS having received the C1 assigns the sub-channel by using the band AMC sub-channel assignment scheme. That is, the second codeword C2 reports to the BS that the SS selects five bandwidths having the best channel conditions and then is to transmit information, i.e., CQI for the selected bandwidths to the BS. That is, the C2 is transmitted to the BS through the CQICH, after the SS transmits the C1.

The third codeword C3 is transmitted from the SS to the BS through the CQICH, which is the dedicated control channel. That is, the third codeword C3 is transmitted to request the BS to convert the sub-channel assignment scheme from the band AMC sub-channel assignment scheme to the normal sub-channel assignment scheme and to assign the sub-channel. That is, the BS, having received the C3 from the SS through the CQICH, converts the sub-channel assignment scheme from the band AMC sub-channel assignment scheme to the normal sub-channel assignment scheme, and then assigns the sub-channel to the SS.

In the sub-channel assignment scheme request to the BS, the SS determines the necessity of the sub-channel assignment scheme conversion according to its own CQI, and transmits the three codewords to the BS through the CQICH according to results of the determination. The BS receives the three codewords through the CQICH, and assigns the sub-channel using either the normal sub-channel assignment scheme or the band AMC sub-channel assignment scheme according to the three received codewords and the CQI based on the user demand. Further, a process in which the SS determines the necessity of the sub-channel assignment scheme conversion will be described in detail with reference to Table 1.

FIG. 4 is a flow diagram illustrating a signal flow for sub-channel assignment between a BS and an SS in a BWA communication system according to an embodiment of the present invention. More specifically, FIG. 4 illustrates a flow diagram when the BS converts a sub-channel assignment scheme from a normal sub-channel assignment scheme to a band AMC sub-channel assignment scheme, and assigns a sub-channel. That is, FIG. 4 illustrates a flow diagram when the BS performs conversion to the band AMC sub-channel assignment scheme in a state in which the BS has assigned the sub-channel to the SS by using the normal sub-channel assignment scheme.

Referring to FIG. 4, it is initially assumed that the SS 410 has received the sub-channel from the BS 420 through the normal sub-channel assignment scheme as described above. Thereafter, when the SS 410 determines that it is necessary to convert the sub-channel assignment scheme through its own CQI, i.e., when the SS 410 determines that it is necessary to convert the sub-channel assignment scheme from the normal sub-channel assignment scheme to the band AMC sub-channel assignment scheme, the SS 410 transmits the first codeword C1 to the BS 420 through a CQICH in step 401. The CQICH is a channel having been already assigned by the BS 420 in order to receive the CQI from the SS 410, and 6 bits are assigned to the CQICH as described above.

The C1 is transmitted from the SS 410 in order to receive the sub-channel from the BS 420 through the band AMC sub-channel assignment scheme converted from the normal sub-channel assignment scheme as described above. That is, the SS 410 transmits the C1 to the BS 420 through the CQICH in order to receive the sub-channel from the BS 420 through the band AMC sub-channel assignment scheme. Because the process in which the SS 410 determines that it is necessary to convert the sub-channel assignment scheme from the normal sub-channel assignment scheme to the band AMC sub-channel assignment scheme will be described in detail with reference to Table 1 below, a detailed description will be omitted here.

The BS 420, having received the C1, transmits a CQI Report-Request (REP-REQ) message to the SS 410 in order to cause the SS 410 to transmit its own CQI in step 402. The SS 410 then measures CINR values of five bandwidths having the best channel conditions in a whole bandwidth, and transmits a Report Response (REP-RSP) message including a CQI having the measured CINR values to the BS 420 through the CQICH in response to the REP-REQ message. Further, the SS 410 transmits the second codeword C2 to the BS 420 through the CQICH in step 403. The second codeword C2 reports that the SS 410 is to transmit information, i.e., the CINR values, for the five bandwidths having the best channel conditions to the BS through the REP-RSP message in order to cause the BS 420 to assign the sub-channel through the band AMC sub-channel assignment scheme. That is, the C2 is reports that the SS 410 is to transmit the CQI, which includes a differential value, i.e., Band AMC Differential, of the CINR values for the five selected bandwidths, in a frame next to the frame through which the REP-RSP message is transmitted. The differential value of the CINR values represents an incremental and decremental value between the CINR values for the five bandwidths selected in a frame through which the REP-RSP message is transmitted and the CINR values for the five bandwidths selected in the next frame after a frame through which the REP-RSP message has been transmitted.

After receiving the REP-RSP message and the C2 from the SS 410 through the CQICH, the BS 420 assigns the sub-channel to the SS 410 through the band AMC sub-channel assignment scheme in step 404. Even when the BS 420 does not receive the REP-RSP message, the BS 420 receives the C2 from the SS 410 through the CQICH, which is a dedicated control channel, thereby recognizing that the SS 410 selects the five bandwidths having the best channel conditions and then transmits both the CINR value for the selected bandwidths and the differential value of the CINR values in the next frame.

After receiving the sub-channel from the BS 420, the SS 410 transmits the CQI, which includes the differential value, i.e., Band AMC Differential CQI, of the CINR values for the five selected bandwidths having the best channel conditions, to the BS 420 in step 405. That is, the SS 410, having received the sub-channel, already knows the CINR value measured from the five selected bandwidths having the best channel conditions in the frame through which the REP-RSP message is transmitted, and measures a CINR value for the five selected bandwidths again in the next frame after the frame through which the REP-RSP message is transmitted. Further, the SS 410 computes a differential value between the measured CINR values for the five selected bandwidths in the two frames, and transmits the differential value to the BS 420.

For example, when the CINR value measured in the frame through which the REP-RSP message has been transmitted is smaller than the CINR value measured in the next frame, i.e., when the CINR value has increased, the SS 410 transmits a CQI having the differential value of 1 to the BS 420. However, when the CINR value measured in the frame through which the REP-RSP message has been transmitted is larger than the CINR value measured in the next frame, i.e., when the CINR value has decreased, the SS 410 transmits a CQI having the differential value of 0 to the BS 420.

After receiving the differential value between the CINR values for the five bandwidths having the best channel conditions from the SS 410 through the CQICH as described above, when the differential value is 1, the BS 420 recognizes that the CINR value of the selected bandwidth has increased. However, when the differential value is 0, the BS 420 recognizes that the CINR value of the selected bandwidth has decreased.

Further, when the SS 410, having received the sub-channel through the band AMC sub-channel assignment scheme from the BS 420, is to receive another sub-channel, i.e., another band AMC sub-channel, through the band AMC sub-channel assignment scheme, the SS 410 transmits the first codeword C1 to the BS 420. That is, when five bandwidths having the best channel conditions in the previous frame are different from that in the current frame, i.e., when the five bandwidths having the best channel conditions change, the SS 410 having already received the sub-channel through the band AMC sub-channel assignment scheme from the BS 420 transmits the first codeword C1 to the BS 420 through the CQICH in order to receive another band AMC sub-channel. Then, the BS 420 having received the C1 transmits the REP-REQ message to the SS 410, and the SS 410 having received the REP-REQ message transmits the C2 and the REP-RSP message to the BS 420 through the CQICH.

The BS 420 assigns a sub-channel to the SS 410 through the band AMC sub-channel assignment scheme, and the SS 410, having received the sub-channel, transmits a CQI, i.e., a Band AMC Differential CQI, including a differential value of CINR values to the BS 420. That is, an operation between the SS 410 and the BS 420, when the five bandwidths having the best channel conditions selected from a whole bandwidth are varied, is the same as when the SS 410 requests conversion to the band AMC sub-channel assignment scheme from the normal sub-channel assignment scheme.

When the CINR value of the selected bandwidth is updated without changing the five bandwidths having the best channel conditions, the SS 410 transmits the C2 and the REP-RSP message to the BS 420, in order to update the CINR value. That is, the SS 410 measures the CINR value of the five selected bandwidth again, and transmits the REP-RSP message including a CQI having the measured CINR value to the BS 420 through the CQICH. Further, the SS 410 transmits a CQI to the BS 420 through the CQICH.

FIG. 5 is a flow diagram illustrating a signal flow for sub-channel assignment between a BS and an SS in the BWA communication system according to the present invention. More specifically, FIG. 5 illustrates a flow diagram when the BS converts a sub-channel assignment scheme from the band AMC sub-channel assignment scheme to the normal sub-channel assignment scheme, and assigns a sub-channel. That is, FIG. 5 illustrates a flow diagram when the BS performs conversion to the normal sub-channel assignment scheme in a state in which the BS has assigned the sub-channel to the SS using the band AMC sub-channel assignment scheme.

Referring to FIG. 5, it is assumed that SS 510 has already received the sub-channel from the BS 420 through the band AMC sub-channel assignment scheme as described above. When the SS 510 determines that it is necessary to convert the sub-channel assignment scheme through its own CQI, i.e., when the SS 410 determines that it is necessary to convert the sub-channel assignment scheme from the band AMC sub-channel assignment scheme to the normal sub-channel assignment scheme, the SS 510 transmits the third codeword C3 to the BS 520 through a CQICH in step 501. The CQICH has been already assigned by the BS 520 in order to receive the CQI from the SS 510, and 6 bits are assigned to the CQICH as described above.

The C3 is transmitted from the SS 510 in order to receive the sub-channel from the BS 420 through the normal sub-channel assignment scheme converted from the band AMC sub-channel assignment scheme as described above. That is, the SS 510 transmits the C3 to the BS 520 through the CQICH in order to receive the sub-channel from the BS 520 through the normal sub-channel assignment scheme. Because the process in which the SS 510 determines that it is necessary to convert the sub-channel assignment scheme from the band AMC sub-channel assignment scheme to the normal sub-channel assignment scheme will be described in detail with reference to Table 1 below, a detailed description will be omitted here.

After transmitting the C3 to the BS 520 through the CQICH, the SS 510 measures CINR values for a whole bandwidth, and transmits a CQI (Regular CQI of whole Bandwidth) including an average value of the measured CINR values for the whole bandwidth in step 502. That is, in the band AMC sub-channel assignment scheme as described in FIG. 4, the SS 510 selects the five bandwidths having the best channel conditions from the whole bandwidth, and transmits the differential value of the CINR values for the selected bandwidths. However, in the normal sub-channel assignment scheme, the SS 510 transmits the average value of the CINR values for the whole bandwidth to the BS 520. Further, the SS 510 repeatedly transmits the C3 and the CQI (Regular CQI of whole Bandwidth) including the average value of the CINR values for the whole bandwidth, i.e., the average value for the whole bandwidth, to the BS 520 through the CQICH until the BS 520 assigns a sub-channel to the SS 510 by using the normal sub-channel assignment scheme in step 503 and 504.

After receiving the C3 and the average value for the whole bandwidth from the SS 510, the BS 520 assigns the sub-channel to the SS 510 by using the normal sub-channel assignment scheme in step 505.

After receiving the sub-channel from the BS 520 through the normal sub-channel assignment scheme, the SS 510 transmits the CQI (Regular CQI of whole Bandwidth) including the average value of the CINR values for the whole bandwidth to the BS 520 in step 506.

Table 1 below shows physical specific channel encoding information of a Uplink Channel Descriptor (UCD) message including information for each channel in the BWA communication system according to the embodiment of the present invention. TABLE 1 Type Name (1 byte) Length Value Initial ranging 150 1 Number of initial ranging CDMA codes codes. Possible values are 0255.a Periodic ranging 151 1 Number of periodic ranging codes CDMA codes. Possible values are 0255.a Bandwidth 152 1 Number of bandwidth request request codes codes. Possible values are 0255.a Periodic ranging 153 1 Initial backoff window size for backoff start periodic ranging contention, expressed as a power of 2. Range: 015 (the highest order bits shall be unused and set to 0). Periodic ranging 154 1 Final backoff window size for backoff end periodic ranging contention, expressed as a power of 2. Range: 015 (the highest order bits shall be unused and set to 0). Start of ranging 155 1 Indicates the starting number, S, of codes group the group of codes used for this uplink. All the ranging codes used on this uplink will be between S and ((S + N + M + L) mod 256). Where, N is the number of initial- ranging codes M is the number of periodic-ranging codes L is the number of bandwidth-request codes The range of values is 0 ≦ S ≦ 255 Permutation base 156 1 Determines the UL_IDcell parameter for the sub-carrier permutation to be used on this uplink channel UL allocated sub- 157 9 This is a bitmap describing the channel is bitmap sub-channel allocated to the segment in the UL, when using the uplink PUSC permutation. The LSB of the first byte shall correspond to sub-channel 0. For any bit that is not set, the corresponding sub-channel shall not be used by the SS on that segment. Optional 158 13 This is a bitmap describing the permutation UL sub-channel allocated to the allocated sub- segment in the UL, when using the channel is bitmap uplink optional PUSC permutation (see 8.4.6.2.5). The LSB of the first byte shall correspond to sub- channel 0. For any bit that is not set, the corresponding sub-channel shall not be used by the SS on that segment. Band AMC 159 1 dB unit Allocation threshold of the maximum of the Threshold standard deviations of the individual band's CINR measurements over time to trigger mode transition from normal sub- channel to Band AMC Band AMC 160 1 dB unit Release threshold of the maximum of the Threshold standard deviations of the individual band's CINR measurements over time to trigger mode transition from Band AMC to normal sub-channel Band AMC 161 1 Frame unit Allocation Timer Minimum required number of frames to measure the average and standard deviation for the event of Band AMC triggering Band AMC 162 1 Frame unit Release Timer Minimum required number of frames to measure the average and standard deviation for the event triggering from Band AMC to normal sub-channel Band Status 163 1 Frame unit Reporting Maximum period between MAX Period refreshing the Band CINR measurements by the unsolicited REP-RSP Band AMC Retry 164 1 Frame unit Timer Backoff timer between consecutive mode transitions from normal sub- channel to Band AMC when the previous request is failed Band AMC Entry 173 1 dB unit Average CINR Threshold of the average CINR of the whole bandwidth to trigger mode transition from normal sub- channel to AMC Safety Channel 165 1 dB unit Allocation Threshold Safety Channel 166 1 dB unit Release Threshold Safety Channel 167 1 Frame unit Allocation Timer Safety Channel 168 1 Frame unit Release Timer Bin Status 169 1 Frame unit Reporting MAX Period Safety Channel 170 1 Frame unit Retry Timer H-ARQ ACK 171 1 1 = one frame offset 2 = two delay for frames offset 3 = three frames UL burst offset CQICH Band 172 1 Frame unit AMC Transition Delay

As shown in Table 1, the present invention defines new fields for conversion between the normal sub-channel assignment scheme and the band AMC sub-channel assignment scheme in the conventional UCD message, and defines in detail fields having an obscure definition in the conventional UCD message.

A newly added field is “Band AMC Entry Average CINR”. The field “Band AMC Entry Average CINR” represents a threshold value for an average CINR value of the whole bandwidth, for conversion to the band AMC sub-channel assignment scheme from the normal sub-channel assignment scheme. That is, when the average CINR value of the whole bandwidth for the sub-channel, which has been assigned to an SS by a BS through the normal sub-channel assignment scheme, exceeds the threshold value defined in the field “Band AMC Entry Average CINR”, the SS requests the BS to convert the sub-channel assignment scheme from the normal sub-channel assignment scheme to the band AMC sub-channel assignment scheme, and to assign the sub-channel. Further, the present invention further includes new fields defined in detail within the conventional field.

First, the field “Band AMC Allocation Threshold” represents a maximum threshold value of a standard deviation value of CINR values measured for each bandwidth, for conversion to the band AMC sub-channel assignment scheme from the normal sub-channel assignment scheme. That is, when the standard deviation value of the CINR values measured for each bandwidth in the whole bandwidth for the sub-channel of an SS, which has been assigned to the SS by a BS through the normal sub-channel assignment scheme, exceeds the threshold value defined in the field “Band AMC Allocation Threshold”, the SS requests the BS to convert the sub-channel assignment scheme from the normal sub-channel assignment scheme to the band AMC sub-channel assignment scheme, and to assign the sub-channel of the SS.

Further, the field “Band AMC Release Threshold” is a field corresponding to the field “Band AMC Allocation Threshold”, which represents the maximum threshold value of the standard deviation value of the CINR values measured for each bandwidth, for conversion to the normal sub-channel assignment scheme from the band AMC sub-channel assignment scheme. That is, when the standard deviation value of the CINR values measured for each bandwidth for the sub-channel, which has been assigned to an SS by a BS through the band AMC sub-channel assignment scheme, exceeds the threshold value defined in the field “Band AMC Release Threshold”, the SS requests the BS to convert the sub-channel assignment scheme from the band AMC sub-channel assignment scheme to the normal sub-channel assignment scheme, and to assign the sub-channel of the SS.

The field “Band AMC Allocation Timer” represents a minimum number of frames required in order to measure both an average value of CINR values for the whole bandwidth and the standard deviation value of the CINR values according to each bandwidth in a process of conversion to the band AMC sub-channel assignment scheme from the normal sub-channel assignment scheme.

Further, the field “Band AMC Release Timer” is a field corresponding to the field “Band AMC Allocation Timer”, which represents a minimum number of frames required in order to measure both the average value of the CINR values for the whole bandwidth and the standard deviation value of the CINR values according to each bandwidth in a process of conversion to the normal sub-channel assignment scheme from the band AMC sub-channel assignment scheme.

The field “Band Status Reporting MAX Period” represents a maximum period for update of bandwidth CINR values measured by an REP-RSP message first generated by an SS. That is, in a state in which the SS receives the sub-channel from the BS through the band AMC sub-channel assignment scheme, when the CINR value of the five bandwidths having the best channel conditions is updated according to a predetermined period without change in the five bandwidths, the SS measures the CINR value to be updated and transmits an REP-RSP message including a CQI having the measured CINR value to the BS, thereby updating the CINR value. Herein, the SS measures the CINR value within the value defined in the field “Band Status Reporting MAX Period”, transmits the measured CINR value to the BS, in order to update the CINR value.

The field “Band AMC Retry Timer” represents a time interval for which the BS waits in order to convert the sub-channel assignment scheme to the band AMC sub-channel assignment scheme from the normal sub-channel assignment scheme, when the previous request message has failed, i.e., when the BS has failed to transmit an REP-REQ message to the SS after receiving the first codeword C1 for requesting the conversion to the band AMC sub-channel assignment scheme from the normal sub-channel assignment scheme.

Hereinafter, conditions of the fields defined in Table 1 for conversion between the normal sub-channel assignment scheme and the band AMC sub-channel assignment scheme in the BWA communication system according to the embodiment of the present invention will be described.

First, a case of conversion to the band AMC sub-channel assignment scheme from the normal sub-channel assignment scheme will be described. When a maximum standard deviation value of CINR values measured for each bandwidth for a sub-channel of an SS, which has been assigned to the SS by a BS through the normal sub-channel assignment scheme, is smaller than the threshold value defined in the field “Band AMC Allocation Threshold”, and when an average CINR value of the whole bandwidth for the sub-channel of the SS is larger than the threshold value defined in the field “Band AMC Entry Average CINR”, the SS transmits the first codeword C1 to the BS through the CQICH in a state in which the SS receives the sub-channel through the normal sub-channel assignment scheme, thereby requesting the BS to convert the sub-channel assignment scheme to the band AMC sub-channel assignment scheme from the normal sub-channel assignment scheme.

That is, in a process in which the SS requests the BS to convert the sub-channel assignment scheme to the band AMC sub-channel assignment scheme from the normal sub-channel assignment scheme, when the maximum standard deviation value of the CINR values of each bandwidth, which is measured for time period required in order to measure the average CINR value of the whole bandwidth for the assigned sub-channel and the standard deviation value of the CINR values measured for each bandwidth, is smaller than a maximum threshold value of the standard deviation value of the CINR values measured for each bandwidth, and when the average CINR value of the whole bandwidth is larger than a threshold value of the average CINR value of the whole bandwidth, the SS transmits the C1 to the BS through the CQICH, thereby requesting the BS to convert the sub-channel assignment scheme to the band AMC sub-channel assignment scheme from the normal sub-channel assignment scheme. Herein, the SS transmits the REP-RSP message as well as the C1 to the BS. The REP-RSP message includes both information for the five bandwidths having the best channel conditions in the whole bandwidth and the CQI having the measured CINR values.

Next, a case of conversion to the normal sub-channel assignment scheme from the band AMC sub-channel assignment scheme will be described. When the maximum standard deviation value of the CINR values measured for each bandwidth during the time period defined in the field “Band AMC Release Timer” is larger than the threshold value defined in the field “Band AMC Release Threshold”, the SS transmits the third codeword C3 to the BS through the CQICH in a state in which the SS receives the sub-channel through the band AMC sub-channel assignment scheme, thereby requesting the BS to convert the sub-channel assignment scheme to the normal sub-channel assignment scheme from the band AMC sub-channel assignment scheme. That is, in a process in which the SS requests the BS to convert the sub-channel assignment scheme to the normal sub-channel assignment scheme from the band AMC sub-channel assignment scheme, when the maximum standard deviation value of the CINR values of each bandwidth, which is measured for time period required in order to measure the average CINR value of the whole bandwidth for the assigned sub-channel and the standard deviation value of the CINR values measured for each bandwidth, is larger than the maximum threshold value of the standard deviation value of the CINR values measured for each bandwidth, the SS transmits the C3 to the BS through the CQICH, thereby requesting the BS to convert the sub-channel assignment scheme to the normal sub-channel assignment scheme from the band AMC sub-channel assignment scheme.

Last, a case in which the five bandwidths having the best channel conditions selected by the SS are varied in a state in which the SS has received the sub-channel through the band AMC sub-channel assignment scheme will be described. When a CINR value of another bandwidth excluding the five bandwidths selected for the band AMC sub-channel assignment scheme is larger than the average CINR value of the whole bandwidth during the time period defined in the field “Band AMC Allocation Timer”, the SS transmits the C1 to the BS through the CQICH, thereby changing the selected bandwidth, similarly to the case of the conversion to the band AMC sub-channel assignment scheme from the normal sub-channel assignment scheme as described above.

As described above, the present invention newly defines and uses undefined codewords from among codewords assigned to a CQICH in a BWA communication system, thereby stably performing conversion between a normal sub-channel assignment scheme and a band AMC sub-channel assignment scheme and assigning a sub-channel. As a result, it is possible to improve an overall reliability of the BWA system.

While the present invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims. 

1. A method for requesting a Base Station (BS) to assign a sub-channel by a Subscriber Station (SS) in a Broadband Wireless Access (BWA) communication system, the BS assigning the sub-channel to the SS by a predetermined scheme, the method comprising the steps of: determining, by the SS, an assignment scheme of a sub-channel currently assigned to the SS; confirming channel quality indicator of the sub-channel assigned by the determined sub-channel assignment scheme; transmitting, to the BS, a codeword representing a sub-channel assignment scheme to be converted, when the SS determines a need to convert the current sub-channel assignment scheme; and receiving a sub-channel through the converted sub-channel assignment scheme according to the transmitted codeword.
 2. The method as claimed in claim 1, wherein the step of transmitting the codeword to the BS comprises the steps of: when the current sub-channel assignment scheme to be converted is a band Adaptive Modulation and Coding (AMC) sub-channel assignment scheme, transmitting a first codeword and a second codeword representing conversion to the band AMC sub-channel assignment scheme; and when the current sub-channel assignment scheme to be converted is a normal sub-channel assignment scheme, transmitting a third codeword representing conversion to the normal sub-channel assignment scheme.
 3. The method as claimed in claim 2, wherein the first codeword is used when the SS requests the BS to convert the sub-channel assignment scheme to the band AMC sub-channel assignment scheme from the normal sub-channel assignment scheme.
 4. The method as claimed in claim 2, wherein the SS transmits the first codeword when the SS measures a standard deviation value of Carrier-to-Interference Noise Ratio (CINR) values for each bandwidth and an average CINR value of a whole bandwidth of the sub-channel currently assigned to the SS, and then determines that the measured standard deviation value of the CINR values for each bandwidth is smaller than a first preset threshold value and the measured average CINR value of the whole bandwidth is larger than a second preset threshold value.
 5. The method as claimed in claim 2, wherein the second codeword is used when the SS reports that the SS will transmit channel quality indicator for a predetermined number of bandwidths to the BS, the predetermined number of bandwidths having best channel conditions in a whole bandwidth of the sub-channel currently assigned to the SS.
 6. The method as claimed in claim 2, wherein the third codeword is used when the SS requests the BS to convert the sub-channel assignment scheme to the normal sub-channel assignment scheme from the band AMC sub-channel assignment scheme.
 7. The method as claimed in claim 2, wherein the SS transmits the third codeword when the SS measures a standard deviation value of CINR values for each bandwidth in a whole bandwidth of the sub-channel currently assigned to the SS, and then determines that the measured standard deviation value of the CINR values for each bandwidth is larger than a first preset threshold value.
 8. The method as claimed in claim 2, wherein the first codeword, the second codeword, and the third codeword are transmitted to the BS through a dedicated control channel assigned in order to transmit channel quality indicator of the SS to the BS.
 9. A method for requesting, by a Subscriber Station (SS), a sub-channel assignment through a second sub-channel assignment scheme, which is different from a first sub-channel assignment scheme, in a Broadband Wireless Access (BWA) communication system including the SS and a Base Station (BS), the BS assigning a sub-channel to the SS through the first sub-channel assignment scheme, the method comprising the steps of: transmitting, by the SS, a first codeword to the BS through a dedicated control channel; transmitting a second codeword and channel quality indicator of the SS to the BS through the dedicated control channel, when a channel quality indicator request message is received from the BS in response to the first codeword; and receiving a sub-channel through the second sub-channel assignment scheme according to the transmitted channel quality indicator and the first codeword.
 10. The method as claimed in claim 9, wherein the first sub-channel assignment scheme corresponds to a normal sub-channel assignment scheme, and the second sub-channel assignment scheme corresponds to a band Adaptive Modulation and Coding (AMC) sub-channel assignment scheme.
 11. The method as claimed in claim 10, wherein the first codeword is used when the SS requests the BS to convert the sub-channel assignment scheme to the band AMC sub-channel assignment scheme from the normal sub-channel assignment scheme.
 12. The method as claimed in claim 10, wherein the second codeword is used when the SS reports that it will transmit channel quality indicator for a predetermined number of bandwidths to the BS, the predetermined number of bandwidths having best channel conditions in a whole bandwidth of the sub-channel assigned to the SS through the first sub-channel assignment scheme.
 13. The method as claimed in claim 10, wherein the dedicated control channel is assigned in order to transmit the channel quality indicator of the SS to the BS.
 14. The method as claimed in claim 10, wherein the step of transmitting the channel quality indicator of the SS to the BS, comprises: transmitting channel quality indicator for a predetermined number of bandwidths to the BS, wherein the predetermined number of bandwidths have best channel conditions in a whole bandwidth of the sub-channel assigned to the SS through the first sub-channel assignment scheme.
 15. The method as claimed in claim 14, further comprising the step of: transmitting, by the SS, a differential value of Carrier-to-Interference Noise Ratio (CINR) values for the predetermined number of bandwidths having the best channel conditions, when the SS receives a sub-channel through the second sub-channel assignment scheme.
 16. The method as claimed in claim 15, wherein the differential value of the CINR values represents an incremental and decremental value between CINR values for the predetermined number of bandwidths having the best channel conditions in a frame through which the second codeword is transmitted and CINR values for the predetermined number of bandwidths having the best channel conditions in a next frame after the frame.
 17. The method as claimed in claim 15, wherein the SS retransmits the first codeword to the BS when the predetermined number of bandwidths having the best channel conditions change due to lapse of a predetermined time period.
 18. The method as claimed in claim 14, further comprising the step of updating the channel quality indicator for the predetermined number of bandwidths having the best channel conditions by retransmitting, by the SS, the second codeword to the BS when a predetermined time period lapses.
 19. The method as claimed in claim 10, wherein the SS transmits the first codeword when the SS measures a standard deviation value of CINR values for each bandwidth and an average CINR value of a whole bandwidth of the sub-channel assigned through the first sub-channel assignment scheme, and then determines that the measured standard deviation value of the CINR values for each bandwidth is smaller than a first preset threshold value and the measured average CINR value of the whole bandwidth is larger than a second preset threshold value.
 20. A method for requesting, by a Subscriber Station (SS), a sub-channel assignment through a first sub-channel assignment scheme, which is different from a second sub-channel assignment scheme, in a Broadband Wireless Access (BWA) communication system including the SS and a Base Station (BS), the BS assigning a sub-channel to the SS through the second sub-channel assignment scheme, the method comprising the steps of: transmitting, by the SS, a codeword to the BS through a dedicated control channel; transmitting channel quality indicator of the SS according to the transmitted codeword; and receiving a sub-channel through the first sub-channel assignment scheme according to the transmitted channel quality indicator and the codeword.
 21. The method as claimed in claim 20, wherein the first sub-channel assignment scheme is a normal sub-channel assignment scheme, and the second sub-channel assignment scheme is a band Adaptive Modulation and Coding (AMC) sub-channel assignment scheme.
 22. The method as claimed in claim 21, wherein the codeword is used when the SS requests the BS to convert the sub-channel assignment scheme to the normal sub-channel assignment scheme from the band AMC sub-channel assignment scheme.
 23. The method as claimed in claim 21, wherein the channel quality indicator includes an average CINR value for a whole bandwidth of the sub-channel assigned through the second sub-channel assignment scheme.
 24. The method as claimed in claim 21, further comprising the step of retransmitting the codeword to the BS when the SS does not receive a sub-channel through the normal sub-channel assignment scheme.
 25. The method as claimed in claim 21, wherein the SS transmits the codeword when the SS measures a standard deviation value of CINR values for each bandwidth in a whole bandwidth of the sub-channel assigned through the second sub-channel assignment scheme, and then determines that the measured standard deviation value of the CINR values for each bandwidth is larger than a first preset threshold value.
 26. A method for assigning, by a Base Station (BS), a sub-channel to a Subscriber Station (SS) through a predetermined scheme in a Broadband Wireless Access (BWA) communication system including the SS and the BS, the method comprising the steps of: receiving, by the BS, a codeword from the SS, the codeword representing conversion from a current sub-channel assignment scheme to another sub-channel assignment scheme; when channel quality indicator of the SS corresponding to the codeword is received, converting the current sub-channel assignment scheme to the another sub-channel assignment scheme according to the channel quality indicator and the codeword; and assigning a sub-channel through the converted another sub-channel assignment scheme.
 27. The method as claimed in claim 26, wherein the step of converting the sub-channel assignment scheme comprises the steps of: converting a normal sub-channel assignment scheme to a band Adaptive Modulation and Coding (AMC) sub-channel assignment scheme, when a first codeword and a second codeword are received; and converting the band AMC sub-channel assignment scheme to the normal sub-channel assignment scheme, when a third codeword is received.
 28. A system for assigning a sub-channel through a predetermined scheme in a Broadband Wireless Access (BWA) communication system including the SS and the BS, the system comprising: a Subscriber Station (SS) for determining an assignment scheme of a sub-channel currently assigned to the SS, confirming channel quality indicator of the sub-channel assigned by the determined sub-channel assignment scheme, transmitting a codeword representing a sub-channel assignment scheme to be converted, when the SS determines a need to convert the current sub-channel assignment scheme, and receiving a sub-channel by the converted sub-channel assignment scheme; and a Base Station BS for receiving the codeword from the SS, converting the current sub-channel assignment scheme according to channel quality indicator and the codeword, and assigning a sub-channel through the converted sub-channel assignment scheme.
 29. The system as claimed in claim 28, wherein the SS transmits a first codeword and a second codeword representing conversion to the band AMC sub-channel assignment scheme when the sub-channel assignment scheme to be converted is a band Adaptive Modulation and Coding (AMC) sub-channel assignment scheme, and transmits a third codeword representing conversion to the normal sub-channel assignment scheme when the sub-channel assignment scheme to be converted is a normal sub-channel assignment scheme.
 30. The system as claimed in claim 29, wherein the first codeword is used when the SS requests the BS to convert the sub-channel assignment scheme to the band AMC sub-channel assignment scheme from the normal sub-channel assignment scheme.
 31. The system as claimed in claim 29, wherein the SS transmits the first codeword after measuring a standard deviation value of Carrier-to-Interference Noise Ratio (CINR) values for each bandwidth and an average CINR value of a whole bandwidth in the whole bandwidth of the sub-channel currently assigned to the SS, and then determining that the measured standard deviation value of the CINR values for each bandwidth is smaller than a first preset threshold value and the measured average CINR value of the whole bandwidth is larger than a second preset threshold value.
 32. The system as claimed in claim 29, wherein the second codeword is used when the SS reports the BS that the SS will transmit channel quality indicator for a predetermined number of bandwidths to the BS, the predetermined number of bandwidths having best channel conditions in the whole bandwidth of the sub-channel currently assigned to the SS.
 33. The system as claimed in claim 29, wherein the third codeword is used when the SS requests the BS to convert the sub-channel assignment scheme to the normal sub-channel assignment scheme from the band AMC sub-channel assignment scheme.
 34. The system as claimed in claim 33, wherein the SS transmits the third codeword after measuring a standard deviation value of CINR values for each bandwidth in a whole bandwidth of the sub-channel currently assigned to the SS, and then determining that the measured standard deviation value of the CINR values for each bandwidth is larger than a first preset threshold value.
 35. The system as claimed in claim 29, wherein the first codeword, the second codeword, and the third codeword are transmitted to the BS through a dedicated control channel assigned in order to the transmit channel quality indicator of the SS to the BS.
 36. The system as claimed in claim 29, wherein, when a channel quality indicator request message is received from the BS in response to the first codeword, the SS transmits channel quality indicator for a predetermined number of bandwidths to the BS, the predetermined number of bandwidths having best channel conditions in a whole bandwidth of a sub-channel currently assigned to the SS.
 37. The system as claimed in claim 36, wherein the SS transmits a differential value of CINR values for the predetermined number of bandwidths having the best channel conditions when the SS receives a sub-channel through the band AMC sub-channel assignment scheme.
 38. The system as claimed in claim 37, wherein the differential value of the CINR values represents an incremental and decremental value between CINR values for the predetermined number of bandwidths having the best channel conditions in a frame through which the second codeword is transmitted and CINR values for the predetermined number of bandwidths having the best channel conditions in a next frame after the frame.
 39. The system as claimed in claim 37, wherein the SS retransmits the first codeword to the BS when the predetermined number of bandwidths having the best channel conditions change due to lapse of a predetermined time period.
 40. The system as claimed in claim 36, wherein the SS retransmits the second codeword to the BS when a predetermined time period lapses, thereby updating the channel quality indicator for the predetermined number of bandwidths having the best channel conditions. 